This invention relates to a method for the production of large substantially flaw-free single crystals of KTiOPO.sub.4 and the Rb, Tl and As analogues thereof.
U.S. Pat. No. 3,949,323 teaches the use of flaw-free optically useful crystals of KTiOPO.sub.4 (KTP) and their analogues in nonlinear optical devices. This patent and European Patent Specification No. 0022193 disclose the preparation of such crystals by hydrothermal processes. Additionally F. C. Zumsteg et al, J. Appl. Phys. 47, 4980 (1986), R. A. Laudise et al, J. Crystal Growth 74, 275-280 (1986) and R. F. Belt et al, Laser Focus Electro-Optics, 110-124 (October 1983) indicate that hydrothermal processes are the preferred methods of growing KTiOPO.sub.4 crystals. The Belt et al article at page 112 and 124 specifically advises against the use of methods other than hydrothermal processes including ordinary flux growth methods.
Due to the requirements for the use of high pressures (on the order of hundreds of atmospheres) and high temperatures in these hydrothermal processes, the equipment required is very costly and may be difficult to manufacture. Further, these hydrothermal processes have the additional disadvantage of providing an undesirably slow rate of crystal growth. Additionally, while crystals of certain desired orientation as large as three or four millimeters have been cut from crystals grown by the use of these processes, it is desirable, for some optical purposes, that oriented crystals of much larger size of the order of centimeters, for example of three or four centimeters, be provided.
Another process for the manufacture of KTiOPO.sub.4 and its analogues is that shown in U.S. Pat. No. 4,231,838. In the process described in this patent, crystal growth is carried out by heating a mixture of MTiOXO.sub.4 and a nonaqueous flux M.vertline.X.vertline.O.vertline. (where M is K, Tl, Rb or mixtures thereof, and X is P or As) or their precursors to produce a nonaqueous melt and then causing crystal growth of MTiOXO.sub.4 by use of a temperature gradient or by slow cooling of the melt at a rate of not greater than 5.degree. C. per hour. Here too, under the conditions described, the problem exists that the crystals produced are of relatively small size, the largest, as shown in Example 5 of this patent, being 15 mm.times.8 mm.times.2 mm.
J. C. Jacco et al 2. Crystal Growth, 76 (1986) pages 484-488 shows crystal growth of KTiOPO.sub.4 by use of various flux growth techniques including seeded growth by slow cooling using a rotating crucible 120 mm in diameter rotatable about its vertical axis. While crystals up to 15.times.15.times.5 mm in size were grown, these crystals were of poor quality containing a large amount of inclusions. Jacco et al show also that by crystal growth by gradient transport good quality crystals containing a minimum amount of inclusions were produced. However, crystals of only up to 5.times.7.times.12 mm in size were able to be grown by this method.
Additionally, the Jacco et al article states that when TSSG (top seeded solution growth) was employed using both gradient and "isothermal conditions" with and without rotation of the seed, multiple nucleation occurred resulting in formation of only needle prisms.
As evidenced in the prior art exemplified by pages 285 and 299 as well as the figures of Elwell et al, Crystal Growth from High-Temperature Solutions, New York, Academic Press 1975, pp. 272, 273, 283-285, 298, 299, the term "isothermal conditions" employed in the Jacco et al article meant that no attempt is made to provide a temperature gradient in the melt as is used in other growing processes. While Jacco et al might have thought that they had established an adequate degree of spatial isothermal conditions in the melt, in the context of the then-existent state of the art, the normal equipment used and processes employed inevitably created temperature gradients in the melt of the order of 20.degree. C. per centimeter or more. Since none of the workers in the art had linked this small temperature difference to the development of spurious nucleation in the melt, it was considered to be "isothermal conditions". This meaning of the term "isothermal conditions" as used in the Jacco et al article is also evidenced by R. A. Laudise--The Growth of Single Crystals (1970), Prentice-Hall, (Englewood Cliffs N.J.), page 258. It is also noted that the TSSG growth process described by Jacco et al involved positioning of the seed at the melt surface.